Assessments of Oxygenation

Published on 07/03/2015 by admin

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Last modified 07/03/2015

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Chapter 4 Assessments of Oxygenation

Asheesh Kumar, MD , Michael E. Canham, MD , Ulrich H. Schmidt, MD, PhD

1 What do arterial blood gas (ABG) instruments measure?

The current ABG instruments measure pH, PCO₂, and PO₂ using three separate electrodes. The blood gas samples are placed through an inlet into a temperature-controlled chamber (usually 37° C) where the blood is exposed to these three electrode tips. Newer ABG machines have simultaneous CO-oximeter measurements as well. Consequently, with use of an absorption spectrophotometer, measurements of reduced hemoglobin, oxyhemoglobin, carboxyhemoglobin, and methemoglobin can also be made. Often, hemoglobin and basic electrolytes can be measured simultaneously.

2 When should ABGs be used?

Analysis of ABGs is used extensively in the critical care setting to evaluate both acid-base status and oxygen and carbon dioxide gas exchange. Analysis of ABGs can be useful in virtually any critical care situation, especially with regard to the acid-base status, adequacy of perfusion, and adequacy of gas exchange.

3 What alternatives exist to measure respiratory gases and gas exchange?

ABGs, specifically acid-base status, are useful for evaluation of systemic pH, especially in the setting of significant metabolic acidoses. However, alternative mechanisms are clinically available to measure systemic oxygen and carbon dioxide levels.

Pulse oximetry: Pulse oximetry is a noninvasive, continuous method of measuring the saturation of hemoglobin by using the differential absorption of different wavelengths of light depending on the loading conditions of oxygen that can significantly reduce the number of ABGs measured in critically ill patients. Further, recent studies have validated the use of respiratory variation in pulse oximetry to detect fluid responsiveness in critically ill patients.

End-tidal carbon dioxide: In patients without significant pulmonary pathologic conditions, an end-tidal carbon dioxide monitor can approximate the plasma levels of carbon dioxide, indicating ongoing metabolic production and adequacy of ventilation.

Bicarbonate: Levels of serum bicarbonate can provide guidance about serum levels of carbon dioxide and pH, especially in the chronic setting. Elevations of serum bicarbonate can indicate chronic retention of carbon dioxide.

Exhaled carbon dioxide: The content of exhaled carbon dioxide over a given period of time can be measured by several commercially available devices. This can provide information regarding total body carbon dioxide production, as well as the adequacy of carbon dioxide elimination. Specifically, with use of the Bohr equation, the dead-space fraction can be calculated to assess the adequacy of ventilation and underlying degrees of pulmonary pathologic conditions.

Calculated minute ventilation: The overall minute ventilation can be calculated by multiplying the tidal volume by respiratory rate. Again, the minute ventilation can provide information about metabolic demands and production.

4 What is the alveolar gas equation?

The alveolar gas equation is a formula used to approximate the partial pressure of oxygen in the alveolus (PAO₂).

where PB is the barometric pressure, PH₂O is the water vapor pressure (usually 47 mm Hg), FiO₂ is the fractional concentration of inspired oxygen, and R is the gas exchange ratio. (The rate of CO₂ production to O₂ utilization is usually approximately 0.8 at rest). For example, at sea level:

As demonstrated by the alveolar gas equation, significant alterations of other values can affect the alveolar oxygen level. For example, significant elevation of plasma carbon dioxide can lead to a decrease in the partial pressure of alveolar oxygen. Further, changes in the respiratory quotient (e.g., supplemental nutrition with high carbohydrate content) can further decrease alveolar oxygenation because of a high CO₂ production–to–O₂ utilization ratio.

5 What is the alveolar-arterial PO₂ difference, or A − a gradient (A − aO₂ gradient)?

After calculation of the PAO₂ from the alveolar gas equation, the A − aO₂ gradient can be obtained by subtracting the PaO₂ measured from the ABG. The alveolar-to-arterial (A − aO₂) oxygen gradient is the difference between the amount of the oxygen in the alveoli (PAO₂) and the amount of oxygen dissolved in the plasma (PaO₂), as measured by ABG. For example, at sea level (with PCO₂ − 40, PO₂ − 92):

The normal A − aO₂ gradient is dependent on age, body position, and nutritional status. A normal A − a gradient for a young adult nonsmoker breathing air is between 5 and 10 mm Hg. Normally, the A − a gradient increases with age. For every decade a person has lived, his or her A − a gradient is expected to increase by 1 mm Hg. The A − aO₂ gradient is widened under normal conditions by age, obesity, fasting, supine position, and heavy exercise. One predictive equation for estimating PO₂ (at PB = 760 mm Hg) in relation to age is as follows: PO₂

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